The combination of hematoporphyrin derivatives and visible red light has been shown to be an effective treatment for curing several types of spontaneous and transplanted rodent tumors. We have found, using an in vitro model, that the effective cytotoxic agent in this process is probably a short-lived excited electronic state of oxygen, termed singlet oxygen. When effective singlet oxygen traps are incorporated into cells the usual destructive effect of hematoporphyrin and light is eliminated. Further, we find that the efficiency of formation of singlet oxygen within the cells is approximately 16% compared to 76% in solution. Based upon these various measurements we estimate that generation of approximately 10 to the 9th power singlet oxygen molecules per cell are required for effective tumor cell inactivation. BIBLIOGRAPHIC REFEENCES: Photoradiation Therapy II. Cure of Animal Tumors with Hematoporphyrin and Light. T.J. Dougherty, G.B. Grindey, R. Fiel, K.R. Weishaupt and D.G. Boyle. J. Natl. Cancer Inst. 55:115, 1976. Identification of Singlet Oxygen as the Cytotoxic Agent in Photo-inactivation of a Murine Tumor. K.R. Weishaupt, C.J. Gomer and T.J. Dougherty, Cancer Res. 36: July, l976, in press.